mountain alder vs alder tree

mountain alder and alder tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
mountain alder	Yes	No	No	No
alder tree	Yes	No	No	No

As nouns, alder tree is a hypernym of mountain alder; that is, alder tree is a word with a broader meaning than mountain alder:

mountain alder: tree of western United States
alder tree: north temperate shrubs or trees having toothed leaves and conelike fruit; bark is used in tanning and dyeing and the wood is rot-resistant

Other hypernyms of mountain alder include alder.

mountain alder (noun)	alder tree (noun)
small shrubby maple of eastern North America; scarlet in autumn	north temperate shrubs or trees having toothed leaves and conelike fruit; bark is used in tanning and dyeing and the wood is rot-resistant
tree of western United States